The present invention relates to the stabilization of silanol compounds, and more particularly, to a method of stabilizing silanol hydrolysates of organosilanes and the stabilized silanol hydrolysates of organosilanes formed thereby.
The stability of the silanol hydrolysates of hydrolyzable organosilanes formed from silanes which have two or more hydrolyzable groups, especially those having a high silanol content, is particularly low. It has been observed that certain commercial polymers which are silanol hydrolysates of methyltrialkoxysilane show a gradual increase in molecular weight which limits the shelf life to about 3 months at 40.degree. C. when the silanol compound is in solution.
The silanol hydrolysates made from silanes having two or more hydrolyzable groups, are difficult to stabilize, and it is believed that they have a very short shelf life due to the continuing condensation of silanol groups with other silanol groups, of silanol groups with alkoxy groups, and the like. Because of this reactivity of the silanol compounds having a high silanol content, the molecular weight of the compounds increases resulting in increased viscosity and/or gelation within relatively short periods of time.
One method of increasing the stability of these compounds having high silanol content, has been to reduce the acid content of the silanol compound. In prior art processes, this has been accomplished by subjecting the silanol compound to several reflux-washings with water after the hydrolysis of the hydrolyzable organosilane. The final acid level in the silanol hydrolyzate is generally between 0 and about 0.1 ppm hydrogen chloride, and even at these acid levels, the silanol hydrolysates of the hydrolyzable organosilanes show a gradual increase in molecular weight of the silanol compounds maintained at room temperature.
The acid is present in the silanol hydrolysates of organosilanes because of the methods used in making the silanol hydrolysates of hydrolyzable organosilanes. They are generally prepared in an acid medium. The usual method of hydrolyzing organosilanes is described throughout the prior art including U.S. Pat. No. 2,719,859 and U.S. Pat. No. 3,364,246. The usual method of hydrolyzing organosilanes is by means of water maintained at an acidic pH. For example, when an alkoxysilane is hydrolyzed with water, an acid, such as a halogen acid or acetic acid, is dissolved in the hydrolyzing medium, and the hydrolysate product is formed. In other instances, for example, when organohalosilanes are being hydrolyzed, halogen acid is produced as a reaction by-product, and the halogen acid dissolves in the hydrolyzing medium and steadily increases as the hydrolysis proceeds. In the prior art methods, reflux-washings with water have been used to eliminate or substantially reduce the acid content of the silanol hydrolysates of hydrolyzable organosilanes, however, it has been noted that the molecular weight of such silanol hydrolysates, especially those formed from silanes having an average of three or more hydrolyzable substituents, increases with time and thereby reduces the shelf life of the particular silanol compound.
Other methods have also been used to produce stable high silanol resins of chlorosilane hydrolysates and alkoxysilane hydrolysates. They include the process of U.S. Pat. No. 3,861,939 wherein the chlorosilane hydrolysis is carried out in a two phase aqueous/organic solvent system, and the acid-containing aqueous layer is separated from the siloxane resin solution as soon as the hydrolysis is complete. Although this method reduces the contact between the silanol-rich siloxane and the acid, it does not reduce it sufficiently for the controlled hydrolysis of alkyl trichlorosilanes (RSiCl.sub.3). According to U.S. Pat. No. 3,861,939, further acid reduction is obtained by filtering the medium through diatomaceous earth thereby resulting in greater stability. In U.S. Pat. No. 3,489,782 the aqueous phase is separated from the organic phase followed by neutralization with a neutralizing agent, such as, sodium carbonate, and an aqueous wash of the organic phase resulting in a stable resin having a high silanol content from the hydrolysis of alkyl trichlorosilanes (RSiCl.sub.3). In U.S. Pat. No. 3,135,713, neutralizing agents, for example ammonium carbonate, are used for the neutralization of excess by-product acid in making chlorosilane hydrolysates. However, it is difficult to filter out completely such neutralization by-products as ammonium chloride. Shelf stability of chlorosilane hydrolysates having moderate or intermediate silanol levels has been improved by removing all solvent and grinding the resin to a fine powder in the process disclosed in U.S. Pat. No. 3,450,672.
The stability of alkoxysilane hydrolysates is increased in U.S. Pat. No. 3,364,246, cited above, by the hydrolysis of an emulsion of the alkoxysilane in water and in U.S. Pat. Nos. 3,389,114 and 3,397,117 by using a very low level of acid and obtaining the resin as a solid. Although each of the foregoing prior art references suggests a method of stabilizing silanol hydrolysates of various organosilanes, it is desirable to provide improved methods of stabilizing such compositions.